A simple centered cover for a booklet

\setuppapersize[A5][A5]\setuppagenumbering[alternative=doublesided]\starttext\startpagemakeup[align=center,doublesided=no,%
pagestate=start]% we want to number this{\bfc This should be the long title, Andreas' solution\par}\stoppagemakeup
This is the colofon
\chapter{Start}\input knuth
\stoptext

Or, another solution (which can be tuned at will), which relies on a
new feature in MkIV: there is also a layout with the same name as the
makeup environment and you can control the margins of your new title
page environment with \setuplayout[makeupname][...].

\setuppagenumbering[alternative=doublesided]%\showframe\setuppapersize[A5][A4,landscape]\setuparranging[2UP]\definemakeup[titlepage][pagestate=start,doublesided=no]\setuplayout[titlepage][backspace=2cm,width=108mm]% 148mm is the width, 20 mm of backspace,% 20 mm of outer margin\starttext\starttitlepagemakeup\raggedcenter\bfc This should be the long, long, very long title\par\stoptitlepagemakeup
This is the colofon
\chapter{Start here on page 3}\dorecurse{20}{\input tufte\par}\stoptext

Scaled Full Page Image 1

An elegant option is to extend the makeup mechanism to accept the background key: